Conventional technologies to configure and manufacture high voltage semiconductor power devices are still confronted with difficulties and limitations to further improvement of device performances due to different tradeoffs. In vertical semiconductor power devices, there is a tradeoff between the drain to source resistance, i.e., on-resistance, commonly represented by RdsA (i.e., Rds X unit Area) as a performance characteristic, and the breakdown voltage sustainable of the power device. For the purpose of reducing the RdsA, an epitaxial layer is formed with higher dopant concentration. However, a heavily doped epitaxial layer also reduces the breakdown voltage that can be sustained by the semiconductor power device.
Several device configurations have been explored in order to resolve the difficulties and limitations caused by these performance tradeoffs. FIG. 1A illustrates a conventional a power transistor in the form of a P-channel trench metal-oxide-semiconductor field effect transistor (MOSFET) 100. The MOSFET 100 is formed in a P-type semiconductor substrate 101, which acts as the drain of the MOSFET 100. A p-type epitaxial region 102, also known as a drift region, is formed on an upper portion of the substrate 101. An n-type body region 106 is formed on or in the drift region 102, forming the body of the MOSFET 100. A trench 107 is formed within the body region 106 and drift/epitaxial region 102. An insulated gate structure is formed in a trench 107 having a bottom in the drift region and opposing sidewalls extending adjacent the drift region for modulating the conductivity of the channel and drift regions in response to the application of a turn-on gate bias. The insulated gate structure includes an electrically conductive gate electrode 104 in the trench 107 and a dielectric material 109, which is also called a gate oxide (Gox), lining a sidewall of the trench adjacent the channel and drift regions. The gate electrode 104 is insulated from adjacent regions. A P+ source region 108 is formed within a top layer of the body region 106. However, to achieve high breakdown voltage, the drift region doping concentration need to be low enough, this result in high resistance at the p-n junctions between the n-type body layer 106 and the p-type substrate 102 is high, and hence there is high RdsA for devices formed this way.
To reduce RdsA and increase breakdown voltage VBD, shielded gate trench (SGT) MOSFETs are preferred for certain applications over conventional trench MOSFETs because they provide several advantageous characteristics. FIG. 1B shows the cross section of a p-channel SGT MOSFET 150, which includes a p-type substrate 101 such as silicon that acts as the drain, a p-type epitaxial or drift region 102, and an n-type body region 106, which can be similar in configuration to corresponding features in FIG. 1A. A trench 157 is formed within the body region 106 and drift/epitaxial region 102 and extends to the bottom of the epitaxial region 102. A shield electrode 152 typically composed of polysilicon, also called poly 1, is deposited within the trench 157 and is insulated from adjacent regions by dielectric material 160, which is also called a liner oxide (liner OX). A gate electrode 154, which when made of polysilicon is commonly called poly 2, is deposited within the trench 157 and above the shield electrode 152. The gate electrode 154 is insulated from adjacent regions by a thin dielectric material 159, also call a gate oxide (Gox). A P+ source region 108 is formed within a top portion of the body region 106. When a positive voltage is applied to the gate electrode 154, the MOSFET device 150 turns on and a conducting channel is formed vertically within the body region 106 between the source 108 and the drift/epitaxial region 102 along the walls of the trench 157.
Shielded gate trench MOSFETs exhibit reduced on-resistance RdsA and increased breakdown voltage of the transistor. For conventional trench MOSFETs, the placement of many trenches in a channel, while decreasing the on-resistance, also increases the overall gate-to-drain capacitance. The introduction of the shielded gate trench MOSFET structure remedies this issue by shielding the gate from the electric field in the drift region (drain). The shielded gate trench MOSFET structure also provides the added benefit of higher doping concentration in the drift region for the device's breakdown voltage, and hence a better tradeoff between BV and RdsA.
Although the SGT provides advantages, the process of manufacturing SGT MOSFET devices is more complicated because it requires a dual poly process, in which the step of etching back the shield electrode, or poly 1, is difficult to control. In addition, one extra mask is also required for poly 1 linkup. Furthermore, the SGT MOSFET structure presents challenges in forming the dielectric isolation between the shield electrode and gate electrode.
It is within this context that embodiments of the present invention arise.